As photovoltaic solar cell technology is adopted as an energy generation solution on an increasingly widespread scale, fabrication and efficiency improvements relating to solar cell efficiency and current/carrier collection and metallization are required. Efficient solar cell metallization requires base metallization contacting base regions of the solar cell and base busbar(s) collecting current from the base metallization as well as emitter metallization contacting emitter regions of the solar cell and emitter busbar(s) collecting current from the emitter metallization. These basic metallization structures are applicable to various types of front and back contact solar cells including front and back junction back contact solar cells.
With reference to interdigitated back contact (IBC) back junction solar cells, single level metallization structures provide on-cell base and emitter metallization in the form of interdigitated base and emitter metallization fingers as well as on-cell base and emitter busbars, often positioned around the peripheral edge of the solar cell. However, these busbars often result in increased lateral path distance for carrier collection and thus so-called electrical shading due to loss of minority carriers prior to collection by emitter junction—for example an on-cell base busbar does not allow contact to solar cell emitter regions over the cell surface area it occupies, thus forcing the minority carriers to travel laterally within the solar cell (e.g., in silicon) past the full extent of the base busbar surface area to a corresponding nearest emitter metallization finger (in the described case a minority carrier such as a hole (+) in n-type silicon under the base bar must travel within the absorber to an emitter finger). In other words, in prior art embodiments the base busbar does not allow contact to the cell emitter, forcing minority carriers to travel laterally past the full extent of the base busbar to be collected by the nearest emitter junction regions. In practice, and depending on current collection requirements and metallization materials in an IBC solar cell, base and emitter busbars are usually many times larger than corresponding IBC base and emitter fingers (in terms of lateral dimensions and areas). Thus, these known designs result in significant lateral path distance for carriers located under opposite polarity busbars and reduce the semiconductor area available for active current collection, resulting in solar cell efficiency loss due to electrical shading. The electrical shading induced loss of current from active semiconductor material poses a substantial challenge for improving solar cell efficiency.
Further, multi-level solar cell metallization structures often rely on conductive vias/posts/paths for electrical connection between metallization layers. In the case of an interdigitated back contact solar cell having a dual level metallization pattern with second level metallization base and emitter busbars (in other words off-cell busbars), the required conductive current paths from the on-cell base and emitter metallization to the second level metallization may impose current collection restrictions and constraints in the cell regions below the second level metallization busbars and result in the loss of carrier collection. For example, busbars (such as edge busbars positioned peripherally around the solar cell edge) may rely on conductive posts or conductive via plugs formed through an insulating layer for connection to the underlying on-cell metallization. When large area busbars are required, opposite polarity carrier collection of the cell area “covered” by the busbar may be partially or fully lost due to electrical shading associated with the solar cell busbars.